Acoustic resonators can be used to implement signal processing functions in various electronic applications. For example, some cellular phones and other communication devices use acoustic resonators to implement frequency filters for transmitted and/or received signals. Several different types of acoustic resonators can be used according to different applications, with examples including bulk acoustic wave (BAW) resonators such as thin film bulk acoustic resonators (FBARs), coupled resonator filters (CRFs), stacked bulk acoustic resonators (SBARs), double bulk acoustic resonators (DBARs), and solidly mounted resonators (SMRs). An FBAR, for example, includes a piezoelectric layer between a bottom (first) electrode and a top (second) electrode over a cavity. BAW resonators may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs operating at frequencies close to their fundamental resonance frequencies may be used as a key component of radio frequency (RF) filters and duplexers in mobile devices.
An acoustic resonator typically comprises a layer of piezoelectric material sandwiched between two plate electrodes in a structure referred to as an acoustic stack. Where an input electrical signal is applied between the electrodes, reciprocal or inverse piezoelectric effect causes the acoustic stack to mechanically expand or contract depending on the polarization of the piezoelectric material. As the input electrical signal varies over time, expansion and contraction of the acoustic stack produces acoustic waves that propagate through the acoustic resonator in various directions and are converted into an output electrical signal by the piezoelectric effect. Some of the acoustic waves achieve resonance across the acoustic stack, with the resonant frequency being determined by factors such as the materials, dimensions, and operating conditions of the acoustic stack. These and other mechanical characteristics of the acoustic resonator determine its frequency response.
FIG. 1 is a cross-sectional view of a conventional acoustic resonator device, fabricated in accordance with a conventional method of fabrication.
Referring to FIG. 1, acoustic resonator device 100 includes a substrate 105, which defines a cavity 110 enabling acoustic reflection. A bottom electrode 120 is formed on the substrate 105 over the cavity 110. A piezoelectric layer 130 is formed on the substrate 105 and the bottom electrode 120, and a top electrode 140 is formed on a portion the piezoelectric layer 130 that extends over the bottom electrode 120. The bottom electrode 120, the portion of the piezoelectric layer 130 extending over the bottom electrode 120, and the top electrode 140, define an active area of the acoustic resonator device 100.
Notably, the bottom electrode 120 extends beyond the outer edge of the cavity 110, but does not extend to the (connecting) edge the substrate 105. Therefore, when the piezoelectric layer 130 is applied, it spans two levels, including a transition portion where the piezoelectric material transitions between a top surface of the substrate 105 to a top surface of the bottom electrode 120. This configuration increases the chances of defects being formed in the piezoelectric layer 130, such as cracks and voids, particularly for a relatively thick bottom electrode 120. Also, such defects effectively lower the breakdown voltage of the piezoelectric layer 130, enabling the piezoelectric layer 130 to withstand less electro-static discharge (ESD).